For what it's worth, I did a little research on different 'laceweight' yarns. Interestingly, the term true laceweight doesn't seem to have a standard. When I googled the term, I got tons of hits, especially from spinners talking about spinning a 'true laceweight'. There didn't seem to be a consistent or standard ratio of length to weight.
The Yarn Standards weight table doesn't even give a standard for laceweight. The table stops at 'light' which is described as sock, fingering or baby weight.
The Wooly Wonka laceweight, at 1,200 yards and 6 ounces, weighs in at 200 yards per ounce. The Blue Moon Fiber Arts' Laci, the one I'm using for the Bee Fields, comes in an 8 oz. skein and has approximately 1,750 yards, or about 219 yards per ounce. Not a huge difference, but significant if you're running short and are approaching the end of your knitting!
Jaggerspun Zephyr Wool-Silk laceweight has 315 yards per ounce as does Superfine Merino. Suri Alpaca has a whopping 496 yards per ounce.
Correction to post: Anne's Wing 'o the Moth shawl was knitted using K1C2 Douceur et Soie which, like Rowan's Kidsilk Haze, has about 260 yards per ounce (227 meters/25 grams is the put-up for Kidsilk Haze). Thanks to eagle-eyed Mary for noticing my mistake!
It becomes pretty obvious that there's a lot of wiggle room when you call a yarn 'laceweight'. Now, I don't think anyone is going to want to knit a design like Bee Fields with a worsted weight wool, but it seems to me that if you have enough yardage (not weight) of a lighter weight yarn that you like, well then, knit on! :)